1. Field of the Invention
This invention relates to a cassette for housing a stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-ray, .alpha.-ray, .beta.-ray, .gamma.-ray, cathode rays or cathod-rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
The stimulable phosphor sheet used in the aforesaid radiation image recording and reproducing system is usually handled in the form housed in a cassette. Specifically, at the radiation image recording step, the stimulable phosphor sheet is used in the form housed in the cassette and is exposed to a radiation passing through an object and the cassette to have a radiation image of the object stored therein. After the radiation image recording step, the stimulable phosphor sheet is conveyed to a predetermined sheet taking-out position in the form housed in the cassette.
The cassette must be fabricated so that it removably houses the stimulable phosphor sheet and maintains the housed sheet in the condition shielded from light. This is because the stimulable phosphor sheet is loaded into the cassette and used in this form for image recording, and read-out of the image stored in the stimulable phosphor sheet is carried out by taking the stimulable phosphor sheet out of the cassette and exposing the sheet to stimulating rays which cause the sheet to emit light in proportion to the stored radiation energy. Further, the stimulable phosphor sheet must be maintained in the condition shielded from light between the image recording step and the image read-out step so that the sheet is not exposed to light which cause the sheet to emit light prior to image read-out.
FIG. 1 is a partially cutaway perspective view showing the conventional cassette for housing a stimulable phosphor sheet. The cassette comprises a front cover 1 and a rear cover 2 openably joined by a hinge 3, and resilient cushioning materials 4 and 5, which are made of a foamed synthetic resin material, a foamed synthetic rubber material or the like, are respectively secured to the inner surfaces of the front cover 1 and the rear cover 2. The cushioning material 4 secured to the inner surface of the front cover 1 has a recess portion 7 for receiving a stimulable phosphor sheet 6.
FIG. 2 is a schematic view showing the manner in which the cassette is used for recording a radiation image. In the radiation image recording step, the cassette housing the stimulable phosphor sheet 6 and closed by a latch 22 so that the front cover 1 and the rear cover 2 do not open is positioned as shown in FIG. 2. A radiation emitted by a radiation source 20 passes through an object 21 and then the front cover 1. Thus the stimulable phosphor sheet 6 is exposed to the radiation to have a radiation image of the object 21 stored in a stimulable phosphor layer 6a overlaid on a substrate 6b of the stimulable phosphor sheet 6.
The cushioning materials 4 and 5 secured to the inner surfaces of the front cover 1 and the rear cover 2 of the cassette exhibit adequate resiliency and hold the stimulable phosphor sheet 6 therebetween at a predetermined pressure so that the sheet 6 does not move in the cassette. Also, the cushioning materials 4 and 5 protect the stimulable phosphor sheet 6 so that the surface of the sheet 6 does not come into contact with the inner surface of the front cover 1 or the rear cover 2 of the cassette and is not scratched thereby when the sheet 6 is inserted into or taken out of the cassette.
FIG. 3 is a schematic view showing the manner in which the stimulable phosphor sheet 6 is inserted into the cassette. The cassette is positioned so that the front cover 1 having the sheet receiving recess portion 7 faces down, and the rear cover 2 is opened upwardly. The stimulable phosphor sheet 6 is delivered and inserted in the direction as indicated by the arrow by use of a sheet loader using rollers 30 or the like. In this case, no problem arises if the stimulable phosphor sheet 6 is always delivered in the correct direction as indicated by the arrow. However, actually, the sheet delivery direction changes due to various causes, for example, as indicated by the chain line. In this case, an upper corner portion 6c at the front end of the stimulable phosphor sheet 6 comes into contact with the cushioning material 5 inside of the rear cover 2. Since the cushioning material 5 exhibits adequate resiliency, the cushioning material 5 is depressed by the corner portion 6c of the stimulable phosphor sheet 6. As a result, the corner portion 6c is caught by the depressed portion of the cushioning material 5, and the stimulable phosphor sheet 6 cannot be smoothly inserted into the cassette or the sheet insertion becomes impossible.
When the stimulable phosphor sheet 6 is taken out of the cassette, the cassette is positioned with the front cover 1 facing down, and the rear cover 2 is opened upwardly. The stimulable phosphor sheet 6 (not shown in FIG. 3) present in the sheet receiving recess portion 7 inside of the front cover 1 is taken up by use of a sucking disk 31 of an air suction arm. When the surface of the cushioning material 5 inside of the rear cover 2 is smooth and in close contact with the stimulable phosphor sheet 6 without air intervening therebetween, the sheet 6 is moved up in close contact with the cushioning material 5 as the rear cover 2 is opened upwardly for taking out the sheet 6. Or, the stimulable phosphor sheet 6 is slightly raised together with the rear cover 2 and then falls to a position deviated from the sheet receiving recess portion 7. Thus sheet taking out cannot be performed smoothly or becomes impossible.